1. Field of the Invention
The present invention relates, in general, to a catalyst for selective catalytic reduction of nitrogen oxides at high temperature window More specifically, the present invention relates to a preparation of a catalyst for selective catalytic reduction of nitrogen oxides at high temperature window, which is prepared by recycling an alumina-based spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
2. Description of the Prior Art
Generally, nitrogen oxides are found to be a main cause of acid rain and photooxidation negatively affecting the environment, along with hydrocarbon. Now, most countries including Korea strictly forbid the discharge of nitrogen oxides above the allowed standard levels. Accordingly, a technique for removing nitrogen oxides from waste gas in a combustion system has been devised.
Techniques for effectively eliminating nitrogen oxides (NOx) are commonly classified into a selective catalytic reduction (SCR) using a catalyst and a reductant together, a selective non-catalytic reduction (SNCR) using only a reductant without a catalyst, a low-NOx burner technique controlling a combustion state in the burner and so on. Among them, the selective catalytic reduction is valued as an effective technique for removing nitrogen oxides, taking notice of the generation of secondary pollution, removal efficiency, operation cost, etc. By using the selective catalytic reduction technique, nitrogen oxides may be removed with an efficiency of 90% or greater and the endurance period thereof may be used for about 2-5 years. In addition, said technique is technically advantageous because poisonous dioxin may be removed, along with nitrogen oxides, in the incinerator.
Catalysts useful in the selective catalytic reduction are classified into an extruded honeycomb catalyst, a metal plate catalyst, and a pellet catalyst, depending on their external forms. Currently, the extruded honeycomb and the metal plate catalysts are widely used in steam power plants and incinerators. Useful as a support of the catalysts are titania, alumina, silica, zirconia and so on, and the catalyst composition mainly comprises oxides of active metals such as vanadium, molybdenum, nickel, tungsten, iron, and copper, and further comprises other active metal components for broadening temperature ranges and enhancing durability of the catalyst.
It became recently known that a catalyst for selective catalytic reduction can be manufactured containing oxides of crystalline phases by impregnating a support of inorganic oxides such as titania, alumina, silica and zirconia with catalytic components such as vanadium, molybdenum, nickel and tungsten, followed by thermal treatment.
In this regard, U.S. Pat. No. 5,827,489 discloses a process for the preparation of a catalyst for selective catalytic reduction containing oxides of crystal phases by impregnating a support of inorganic oxides such as titania, alumina, silica and zirconia with catalytic components such as vanadium, molybdenum, nickel and tungsten, thereafter heat treating. This patent employs a support and catalytic components with a superior poisoning resistance to sulfur oxides for the selective catalytic reduction and has advantages of freely controlling the amounts of active metals, a specific surface area and pore sizes of the catalyst to prepare the catalyst having optimal performance in which a suitable amount of sulfate is added. On the other hand, it suffers from high preparation cost because each of single materials (or precursors) used as the support and the catalyst should be prepared by methods of catalyst production and mixing.
Thus, the catalyst for selective catalytic reduction of nitrogen oxides may be prepared to show a catalytic activity at low, medium or high-temperature window by freely selecting a support with poisoning resistance to sulfur oxides, moisture and dusts, an impregnated amount of active metals, and a specific surface area and pore size of the support. For example, platinum-based catalysts are used at low temperature, vanadium-impregnated titania catalysts at medium temperature and zeolite catalysts at high temperature.
In the case of a catalyst at high temperature window for the selective catalytic reduction, it is commonly required to have a large specific surface area and contain limited amounts of active metals therein. A large specific surface area is intimately associated with the crystalline structure of the support. The support having larger specific surface area is exemplified by titania having anatase crystalline structure, alumina having gamma alumina crystalline structure and zeolite such as mordenite. Useful as active metals at high temperature window are vanadium, tungsten, molybdenum and so on, of which vanadium should be used at a suitable amount or less, and molybdenum and tungsten at a suitable amount or more.
In catalysts as described above, the catalyst comprising a titania support having anatase crystalline structure impregnated with vanadium is most preferably used as the catalyst for selective catalytic reduction in terms of performance and durability of the catalyst. However, said catalyst is only usable in limited temperature ranges of 200-400xc2x0 C., preferably 250-350xc2x0 C., which is attributed to characteristics of vanadium, and specific surface areas and pore sizes of titania support. In particular, titania with an anatase crystalline structure has larger specific surface area due to a great quantity of micropores and is phase-changed into rutile crystalline structure having a specific surface area of 10 m2/g at about 550xc2x0 C. Meanwhile, gamma alumina is limitedly used because of poisoning attributable to sulfur oxides from exhaust gas. However, gamma alumina is phase-changed into a structure having a specific surface area of about 1-5 m2/g at 1000xc2x0 or higher, therefore it is suitable for a support at high temperature window, if poisoning problems are solved. The extruded zeolite-based catalyst, which has good catalytic performance at high temperatures of 400-600xc2x0 C., is not widely used owing to extrusion difficulty attributed to larger specific surface area and poisoning caused by moisture. As it is, the catalyst at high temperature window has been applied to gas turbines, engines for ships, and power plants at temperatures of 500xc2x0 C. or lower,
Meanwhile, oil refineries essentially employ a hydro-desulfurization process for removing sulfur components contained in crude oil, from which a spent catalyst is discharged as a by-product. However, if such a spent catalyst is not recycled, treatment costs therefor are required continuously, which is disadvantageous in the economic aspect.
In this regard, Korean Patent Laid-Open No. 95-72277 and U.S. Pat. No. 6,171,566 refer to recycling of spent catalysts discharged from a hydro-desulfurization process of an oil refinery. A catalyst for selective catalytic reduction prepared by recycling such spent catalysts is more advantageous in terms of low preparation cost, inherent poisoning resistance to sulfur oxides, and containing the high content of metal components with excellent activities for nitrogen oxides reduction, compared with a catalyst prepared by a combination process of single materials. However, spent catalysts usable in the preparation of the catalyst for selective catalytic reduction may be recycled with only a 30% recovery rate on the basis of the whole discharged amounts of the whole spent catalysts, and thus intensive research for solving said problems has been carried out.
Leading to the present invention, the intensive and thorough research on a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding that a spent catalyst comprising an alumina support (preferably gamma alumina) with a large specific surface area impregnated with low contents of vanadium and high contents of molybdenum may be recycled to prepare a catalyst for selective catalytic reduction of nitrogen oxides. When used in selective catalytic reduction in the presence of ammonia as a reducing agent, the thusly prepared catalyst has more excellent selective removal activity of nitrogen oxides at high temperature window by containing suitable amounts of metal components therein and a better poisoning resistance to sulfur oxides, compared with those of the conventional catalysts.
Therefore, It is an object of the present invention to provide a catalyst having an excellent selective removal activity of nitrogen oxides at high temperature window and a good poisoning resistance to sulfur oxides.
It is another object of the present invention to provide a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides at high temperature window using a spent catalyst discharged from a hydro-desulfurization process of an oil refinery.
In accordance with the present invention, there is provided a method for preparing a catalyst for selective catalytic reduction of nitrogen oxides at high temperature window comprising the following steps:
a) pretreating a spent catalyst discharged from a hydro-desulfurization process of an oil refinery, which comprises 4 wt % or less of vanadium, 4 wt % or less of nickel, 5 wt % or more of molybdenum and 1 wt % or less of sulfur on an alumina support by thermally treating said spent catalyst followed by washing with water;
b) providing a titania impregnated with 3 to 10 wt % of tungsten on the basis of titania weight;
c) pulverizing the pretreated spent catalyst, followed by homogeneously mixing the pulverized spent catalyst with the tungsten-impregnated titania under the addition of water and acid;
d) dehydrating the mixture to remove excess moisture and active metal components therein;
e) drying the dehydrated mixture at 100 to 200xc2x0 C. for 9 hours or longer, followed by grinding the dried mixture; and
f) forming a catalyst body by extruding the grinded mixture or coating the grinded mixture to a structure, followed by drying under a constant temperature and humidity condition and then calcining the dried structure.